For the first time, scientists have combined gravitational wave measurements, telescope observations and collisions of gold atoms in particle accelerators to map the properties of the extremely compact material in neutron stars.
Neutron stars are the small compact remnants of exploded massive stars at least ten times the mass of the Sun. The matter in these stars is so densely packed that there are no longer any atoms consisting of a nucleus and electrons. These are compressed into uncharged nuclear particles called neutrons. A teaspoon of neutron star material weighs millions of tons, as much as a large mountain.
Much is still unknown about this neutron stuff. ‘The deeper you descend into the interior of a neutron star, the higher its density becomes. It is difficult to determine how the matter then looks and behaves’, says physicist Peter Pang | from Utrecht University and one of the first authors of the new research that appeared in the journal Nature. ‘Not only because we cannot simulate such high densities on Earth, but also because it is difficult to calculate this theoretically.’
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To find out the properties of this mysterious stuff, astronomers and physicists have combined observations of particle collisions with gravitational wave measurements for the first time. 2017 and 2019. Both measured spacetime ripples created by colliding neutron stars.
They combined this data with measurements of colliding charged gold atoms, from the particle accelerator at the German research institute GSI Helmholtz Center for Heavy Ion Research in Darmstadt. Densities are achieved that are more than one and a half times higher than the density in an atomic nucleus. This is far from being compared to neutron stars, but it does help to improve the theoretical models.
Finally, the researchers also included measurements from telescopes, such as the NICER telescope aboard the ISS space station. They collected information about eight neutron stars. “Everything we could use to investigate the compact neutron matter has also been used,” says Chris Van Den Broeckfrom Utrecht University and Nikhef.
The analysis of the combined measurements shows that a neutron star with 1.4 times the mass of the Sun has a diameter of 24 kilometers, comparable to the city of Rotterdam. This number is in line with earlier estimates, but it is more accurate.
The researchers also estimated the stiffness of the neutron matter. “The NICER observations told us that the material is stiffer than we expected,” says Pang. ‘This turned out to correspond with the measurements of gold collisions in the particle accelerator.’
The most surprising thing about the analysis, according to Pang, was how well the findings of the different measurements matched. “The particle collision measurements show remarkable consistency with the astrophysical observations, even though they were obtained with completely different measurement methods.”
The results are therefore not really surprising. But this analysis is just the beginning. ‘With this we lay the foundation for the work to come’, says Van Den Broeck. The aim was also to find out how to combine such measurements so that in the future new observations of the LHC particle accelerator at CERN in Geneva and of the future gravitational wave detector, the Einstein Telescope can be added.
‘With these extra observations, we can determine exactly what the properties are of the material in neutron stars,’ says Pang.
Now the researchers can only say something about the pressure and density. But there is still much to discover. The material is thus a liquid, the viscosity of which, or the viscosity, and the conductivity can also be determined. Van den Broeck: ‘I think that combining future measurements will really be a game-changer.’